This invention relates generally to cooling of dynamoelectric machines, and in particular to a fan and baffle for directing a flow of cooling air through an electric motor which ventilates the motor while minimizing the generation of noise.
A substantial concern in the use of electric motors in many appliances is the generation of noise. Motor-induced noise emanating from pumps, blowers, dishwashers, or clothes washers and dryers can be, at best, an annoyance which must be tolerated or, at worst, a major obstacle to the marketability of the appliance. Consumers and appliance manufacturers generally favor motors which are more quiet. Studies for locating the sources of noise generation in motors have found that ventilation systems are a major contributor. Ventilation is necessary because electric motors generate heat during operation which may be detrimental. Component parts such as windings and control boards may be damaged or degraded by high temperatures. Consequently, motors are typically cooled by air moving through and around the motors which transports heat energy away from hot parts.
An internal ventilation system of a motor frequently includes a fan and a baffle for directing the flow of cooling air through the housing or casing shell of the motor. In one particular system, the fan is mounted on the shaft of the rotor at one axial end of the motor and draws air from the axially opposite end of the motor on rotation of the fan. The air travels through the motor to cool the motor and is then pushed outward away from the shell by the fan. Frequently, the fan is a radial flow type fan and has a plurality of straight blades which discharge air radially outwardly. The fan blades extend radially outward from a central hub. The blades are typically flat, rectangular members oriented in planes parallel to the axis of rotation of the fan so that they will push air by centrifugal force. Air moving into a central portion of the fan is pushed outwardly toward a plurality of exit vents provided in the shell.
The baffle is typically a flat, ring-shaped disk that is positioned between the stator and the fan to provide for efficient cooling. It forms a barrier to flow of cooling air along an outer periphery of the interior of the shell, such that the air is directed radially inwardly, generally across stator windings, to a central opening in the baffle. The air then reaches the central portion of the fan and is pushed radially outwardly for delivery out away from the motor.
Unfortunately, the flow path of cooling air has resulted in local regions of turbulence, backflow, and flow separations which elevate noise levels. The baffle is flat, with both of its faces oriented radially, and it forces the flow to make generally sharp turns which generates eddy currents. Although flow downstream of the baffle must move in a direction not only radially outward but also with a downstream, axial component to reach the exit vents, the baffle on its downstream face has no contouring in the axial direction. Thus the flow tends to separate from the downstream face of the baffle. The baffle results in flow being exposed to right angle corner turns, which have a tendency to cause flow separation and an undesirable pressure loss resulting in lower air flow rates and velocity. Further, the fan blades are configured to shear the air, which creates noise. As the fan rotates, each blade moves transverse to the axial motion of air being received into the fan. The shearing action also creates a pocket of low pressure around the hub and turbulence which can produce additional noise.
Baffles of the prior art have additional drawbacks. Typically, the baffle is sized for an interference fit in the motor shell. The baffle is installed by pressing the baffle into the shell with the outer perimeter in tight engagement with the internal wall of the shell. However, when the size dimension of the baffle or shell are inadvertently varied due to manufacturing tolerances, the baffle can be installed in a loose or misaligned condition, or can be damaged when press fit into the shell. Moreover, baffles of the prior art frequently have water flow notches in the outer perimeter which degrade cooling efficiency. The notches prevent accumulation of water in the motor shell when it ingests water, such as when exposed to heavy rain, to avoid contact with electrical components. One of the notches positioned on the lower side functions as a passage for allowing water to flow through the shell to the exit vents. Unfortunately, the notches also permit passage of cooling air which can therefore bypass the windings and fan, leading to air recirculation and loss of efficiency.